Some functions, like convex-hull computation or graph visualization, are implemented in several fashions, using different algorithms, libraries, or visualization engines. The choice between the implementations can't always be made from the sole algorithmical complexity point of view; instead, you are allowed to establish own preferences, based on your subjective taste.
Let's recall that preferences can be manipulated in two ways. Permanent settings introduced by prefer statements in the rule files or prefer user comands, are stored in the user's private file ~/.polymake/prefer.pl. Temporary changes are made by calling user functions with the same names as the top-level labels. They hold until the user script finishes, or the input line in the interactive mode is interpreted completely.
The top-level labels defined in the application topaz are:
default, geomview, javaview, postscript